Antifriction bearing units are already known which comprise an antifriction bearing, and a sensor device and a member to be detected, the sensor device and the member to be detected being mounted on the bearing. Such units are used in railroad cars and motor vehicles for supporting an axle or a rotating shaft for transmitting rotation to the axle and for detecting the rotation, i.e., the speed of rotation of the axle or shaft or the angle of rotation thereof.
The publication of JP-U No. 6-47867 discloses such a sensor-equipped antifriction bearing unit comprising a sensor device which is a rotational speed sensor of the electromagnetic induction type and a member to be detected which is in the form of an annular member (pulser ring) having a specified shape and projections. Further the publication of JP-A No. 11-174069 discloses such a sensor-equipped antifriction bearing unit comprising a sensor device which is a magnetic sensor, and a member to be detected which is an annular magnet (magnetized pulser) having N poles and S poles spaced equidistantly.
Since various items of data are required for controlling motor vehicles, it has been proposed to provide a sensor device on a hub unit comprising a rotation-side raceway member for a wheel to be attached thereto, a fixed-side raceway member to be fixed to the body of the vehicle, and two rows of rolling bodies arranged between the two raceway members. For example, the publication of JP-A No. 3-209016 discloses a sensor-equipped hub unit wherein an annular support member is attached to the inner end face of a fixed-side raceway member and provided with a strain sensor.
With the sensor-equipped antifriction bearing units disclosed in the first- and second-mentioned publications, the rotation-side raceway member needs to have attached thereto the member to be detected although the two units are different as to the presence or absence of magnetic poles. The units therefore have the problem of necessitating an increased number of assembling steps and a larger number of components to result in a higher cost.
In recent years, furthermore, the control means in use for motor vehicles include drive force control means not permitting spinning of the drive wheels when the vehicle is started or accelerated, and braking force control means for suppressing lateral skidding for cornering, in addition to the ABS (antilock brake system) control means. To ensure more accurate control, it has become important to detect data which is usable effectively for these modes of control. However, the sensor-equipped hub unit disclosed in the third-mentioned publication has the problem that since the strain of the annular support member is measured, a great error is involved in determining the ground contact load from the strain measured, consequently making it impossible to calculate the ground contact load accurately from the measurement of the strain sensor.